1. Field of the Invention
This invention relates to an electrophotographic photosensitive member which contains in its outermost surface layer a compound obtained by polymerizing or cross-linking and curing a charge transporting compound having chain polymerizable functional groups, and a process cartridge and an electrophotographic apparatus which have the electrophotographic photosensitive member.
2. Description of the Related Art
In the past, as photoconductive materials used in electrophotographic photosensitive members, inorganic electrophotographic photosensitive members making use of inorganic materials such as selenium, cadmium sulfide and zinc oxide have chiefly been in use. Meanwhile, organic electrophotographic photosensitive members making use of organic materials have energetically been on research and development because their advantages such as high productivity and freeness from environmental pollution have attracted notice, and those having photoconductive properties comparable to those of the inorganic electrophotographic photosensitive members have been discovered in a large number and in recent years have come into main use in place of the inorganic electrophotographic photosensitive members.
These electrophotographic photosensitive members are often used as functionally separated electrophotographic photosensitive members in which a charge generation layer and a charge transport layer are superposed in order to satisfy both electrical and mechanical properties. Here, in order to bring out electrical properties always stably and at high sensitivity at the initial stage of course and also when used for a long time, what are very important are the structure and purity of a charge transporting compound. Meanwhile, as a matter of course, in electrophotographic photosensitive members used repeatedly, electrical and mechanical external forces due to charging, imagewise exposure, development with toner, transfer to paper, cleaning and so forth are directly applied to the surfaces of the electrophotographic photosensitive members, and hence they are required to have durability to such external forces. Stated specifically, they are required to have durability to surface wear and scratches caused by rubbing, durability to surface deterioration due to charging (e.g., a lowering of transfer efficiency and slipperiness), and also durability to the deterioration of electrical properties, such as a lowering of sensitivity and a lowering of potential.
The surfaces of electrophotographic photosensitive members are commonly formed by thin resin layers, and properties of resins are very important. As resins that fulfill the above various conditions to a certain extent, acrylic resins, polycarbonate resins and the like are put into practical use in recent years, but it is not the case that all the properties as stated above are satisfied by these resins. In particular, in order to achieve a high durability of the electrophotographic photosensitive members, it is difficult to say that such resins can have a sufficiently high film hardness. Even where these resins are used as resins for forming surface layers, there has been a problem that the surface layers come to wear when used repeatedly and further come scratched.
Further, because of a demand made in recent years for the achievement of high sensitivity of organic electrophotographic photosensitive members, a low-molecular weight compound such as a charge transporting compound is often added in a relatively large quantity. In such a case, the film strength may greatly lower because of the action of such a low-molecular weight substance that is similar to that of a plasticizer, to bring about the problem that the surface layer comes to wear and comes scratched when used further repeatedly. A problem may also arise such that the above low-molecular weight component charge transporting compound comes unwantedly deposited when electrophotographic photosensitive members are stored over a long period of time, to cause layer separation.
As a measure for solving these problems, an attempt to use a curable resin as a resin for charge transport layer is disclosed in, e.g., Japanese Patent Application Laid-open No. H02-127652. The curable resin is thus used as a resin for charge transport layer and the charge transport layer formed is cured or cross-linked. This brings an enhancement of mechanical strength, and a great improvement in wear resistance and scratch resistance in repeated used. However, even if such a curable resin is used, the low-molecular weight component acts as a plasticizer in the binder resin to the last, and hence the problems of deposition and layer separation as stated above are not fundamentally solved. Also, in the charge transport layer constituted of the charge transporting compound and a binder resin, the dependence of charge transport performance on the resin is so great that, e.g., a curable resin promising a sufficiently high hardness may have no sufficient charge transport performance and the residual potential may be seen to increase when used repeatedly. Thus, this measure has not come up with satisfaction of the both.
In, e.g., Japanese Patent Applications Laid-open No. H05-216249, No. H07-072640 and No. 2004-302451, an electrophotographic photosensitive member is disclosed in which a charge transport layer is incorporated with a charge transporting compound monomer having a carbon-carbon double bond and the carbon-carbon double bond of the charge transporting compound is allowed to react by the energy of heat or light to form a cured film as the charge transport layer. This charge transporting compound is set pendantwise stationary to the polymer backbone skeleton as in the present invention. However, the charge transporting compound has only one polymerizable group and also is blended with a commercially available polyfunctional monomer, followed by curing to form the film. Hence, firstly the charge transporting compound having one carbon-carbon double bond must be used in a certain level of concentration in order to bring out a sufficient charge transport performance. Because of the relation of compatibility with the commercially available polyfunctional monomer, it is also difficult to configure charge transporting materials in the film in a uniform and optimum state. Thus, in the actual circumstances, it is unable to sufficiently secure both the mechanical strength and the charge transport performance. Further, it is concerned that initiators required at the time of polymerization may affect electrophotographic performance, and in practice they affect it to cause an increase in residual potential and potential variations at the time of running to bring about a problem.
As another measure for solution, in, e.g., Japanese Patent Application Laid-open No. H08-248649, an electrophotographic photosensitive member is also disclosed in which a group capable of transporting electric charges is introduced into the backbone chain of a thermoplastic high polymer to form a charge transport layer. This charge transport layer is more effective against the deposition and layer separation than conventional molecule-dispersed type charge transport layers and brings an improvement in mechanical strength as well. However, the high polymer used is a thermoplastic resin to the last. There is a limit to its mechanical strength, and it is difficult to say that such a polymer is satisfactory in respect of handling and productivity, inclusive of solubility and so forth of the resin.
As discussed above, any systems hitherto available have not achieved both the high mechanical strength and the high charge transport performance. Under such circumstances, in various publications, the present inventors have proposed that a charge transporting compound having chain polymerizable functional groups may be cross-linked and cured by irradiation with electron rays or ultraviolet rays or by heat, whereby the above problems can vastly be remedied (see, e.g., Japanese Patent Applications Laid-open No. H11-265085. No. 2000-066424, No. 2000-066425, No. 2000-206715, No. 2000-206716 and No. 2001-166519).